Research Shows High Effectiveness of Mix-and-Match COVID-19 Vaccines

People who received a first dose of the Oxford-AstraZeneca COVID-19 vaccine and received an mRNA vaccine for their second dose had a lower risk of infection compared to people who had both doses of the Oxford-AstraZeneca vaccine got. This is according to a nationwide study conducted by researchers from the University of Umeå, Sweden.

“Having one of the approved vaccines is better than no vaccine, and two doses are better than one,” said Peter Nordström, professor of geriatric medicine at Umeå University. “However, our study shows a greater risk reduction for people who received an mRNA vaccine after receiving an initial dose of a vector-based vaccine, compared to people who received the vector-based vaccine for both doses.”

As the Oxford-AstraZeneca vector-based vaccine against COVID-19 was discontinued for people under the age of 65, all individuals who had already received their first dose of this vaccine were recommended an mRNA vaccine as the second dose.

Professor and senior consultant (accompanying) physician Department of Community Medicine and Rehabilitation, Unit of Geriatric Medicine Umeå University. Credit: Mattias Pettersson

During a mean follow-up period of 2.5 months after the second dose, the study showed a 67% lower risk of infection for the Oxford-AstraZeneca + Pfizer-BioNTech combination and a 79% lower risk for Oxford/AstraZeneca + Moderna, both compared to unvaccinated individuals. For people who had received two doses of the Oxford-AstraZeneca vaccine, the risk reduction was 50%.

These risk estimates were observed after taking into account differences related to vaccination date, age of participants, socioeconomic status and other risk factors for COVID-19. Importantly, the effectiveness estimates apply to infection with the Delta variant, which dominated confirmed cases during the follow-up period.

There was a very low incidence of thromboembolic adverse events for all vaccine schedules. The number of COVID-19 cases severe enough to lead to hospitalization was too low for the researchers to calculate effectiveness against this outcome.

Marcel Ballin PhD student Department of Community Medicine and Rehabilitation, Unit of Geriatric Medicine Umeå University. Credit: Mattias Pettersson

Previous research has shown that mix-and-match vaccine schedules generate a robust immune response. However, it is unclear to what extent these schedules can reduce the risk of clinical infection. This is the knowledge gap that the new study by the Umeå researchers aimed to fill. The study is based on national registry data from the Swedish Public Health Agency, the National Board of Health and Welfare and Statistics Sweden. Approximately 700,000 individuals were included in the main analysis.

“The results of the study may have implications for vaccination strategies in different countries,” said Marcel Ballin, a doctoral student in geriatric medicine at Umeå University and co-author of the study. “The World Health Organization has stated that despite the promising results of previous studies regarding the immune response of mix-and-match vaccination, there is a need for larger studies to investigate their safety and effectiveness against clinical outcomes. Here we now have such an investigation.”

Reference: “Effectiveness of heterologous ChAdOx1 nCoV-19 and mRNA prime-boost vaccination against symptomatic Covid-19 infection in Sweden: a nationwide cohort study” October 18, 2021, The Lancet Regional Health – Europe.
DOI: 10.116/j.lanepe.2021.100249